On-chip transformers are considered to be the primary components in many RF wireless applications. This paper provides an in-depth review of on-chip transformers, starting with a presentation on the various equivalent circuit models to represent transformer behavior and characterize their performance. Next, a comparative study on the different design and layout strategies is provided, and the fabrication techniques for on-chip implementation of transformers are discussed. The critical performance parameters to characterize on-chip transformers, such as the Q-factor, coupling factor (k), resonance frequency (fSR), and others, are discussed with reference to trade-offs in silicon chip real-estate. The performance parameters and area requirements for different types of on-chip transformers are summarized in tabular form and compared. Several techniques for performance enhancement of on-chip transformers, including the different types of micromachining and integration approaches stemming from MEMS (microelectromechanical systems) technologies are also analyzed. Lastly, the different uses and applications of on-chip transformers are discussed to highlight the evolution of on-chip transformer technology over the recent years and provide directions for future work in this field.

This work reports on the modelling and process technology development for the design and fabrication of vertical, 3D, monolithic RF-MEMS inductors based on self-assembly via intrinsic stresses otherwise referred to as residual or internal stresses in thin films. Stressinduced bending in different cantilever designs were modelled at various film thicknesses using finite element analysis method and bending conditions were optimized. Intrinsic stress-induced bending mechanism is verified by fabrication of bi-layer metallic micro cantilever structures with varying stress conditions which reach bending angles of up to 137° and possibly more upon release. By modulating the loading mode (tensile or compressive) along the beam length, complex out-of-plane wavy cantilevers with multiple upward and/ or downward bends were realized. The fabrication and modelling results display large overlap which further demonstrates the applicability of intrinsic stress-induced bending as a controllable technology towards fabrication of out-of-plane 3D micro components. Additionally, as a potential application to RF-MEMS inductors, stress-induced self-assembly of patterned thin film stacks into out-of-plane inductor topologies of varying geometries was investigated. Electromagnetic modelling tools were used to study the effect of bending on inductor performance (primarily the Q factor and self-resonance frequency-f SR ), and results were evaluated by comparison to planar inductors of the same number of turns and dimensions, which revealed Q factor and f SR improvement of more than 100% upon bending away from substrate surface.

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